Fibrillar, nanotextured coating and method for its manufacture

ABSTRACT

A fibrillar, nanotextured coating is deposited on a substrate by contacting the substrate with a reaction mixture comprising a reagent which is hydrolyzable to produced a cross-linked reaction product, and a first solvent which solvates the reagent and the reaction product. The reagent is hydrolyzed so as to provide a cross-linked reaction product which is bonded to the substrate. The substrate is then contacted with a second solvent which is a non-solvent for the reaction product so as to cause nanoscopic phase separation of the reaction product, resulting in the formation of a fibrillar nanotextured coating which is bonded to the substrate. The thus produced coating may be subjected to further chemical modification. The method may be utilized to produce superhydrophobic coatings. Also disclosed are coatings made by the method of the present invention.

RELATED APPLICATION

This application claims priority of U.S. Provisional Patent ApplicationSer. No. 60/748,474 filed Dec. 8, 2005, entitled “First PerfectlyHydrophobic Surface” and is a divisional application of Ser. No.11/481,270 filed Jul. 5, 2006, entitled “Fibrillar, Nanotextured Coatingand Method for Its Manufacture.”

FIELD OF THE INVENTION

This invention relates generally to coatings. More specifically, theinvention relates to a fibrillar, nanotextured coating and to methodsfor its manufacture.

BACKGROUND OF THE INVENTION

The nanotexture of a surface can influence various properties of thatsurface such as its wettability by water and oils, its opticalproperties, and its chemical reactivity. Consequently, the art hassought methods and materials for controlling the nanotextures of variousmaterials. Chemical methods such as etching processes, and physicalmethods such as sandblasting and other erosion processes have beenutilized with success to control the microtexture of various materials.However, such methods have generally been inadequate for providingnanotextured surfaces.

The prior art, as exemplified by U.S. Pat. No. 2,306,222, has recognizedthat particular silane materials may be utilized to deposit awater-repellant coating onto various substrates. The coating depositedby the use of this technology is a relatively smooth coating, andvarious approaches have been implemented to texturize this coating so asto increase its water repellency. For example, U.S. Pat. No. 6,649,266shows the deposition of silane coatings onto microtextured substrates toprovide coatings having enhanced hydrophobicity. Another approach isdescribed in PCT Published Application WO 2005/068399. This publicationdescribes the use of a sol/gel chemical process for depositing asponge-like water-repellant coating having a nanoscale roughness. Use ofthis technology to deposit a coating onto a surface having an additionalmicroscale texture has been found to provide a coating with furtherenhanced hydrophobicity.

Despite various efforts the prior art has not been able to prepare asynthetic surface which is perfectly hydrophobic. As will be explainedhereinbelow, the hydrophobic nature of a surface may be quantified bythe contact angle that surface forms with a droplet of water. Aperfectly hydrophobic surface has a contact angle of 180 degrees, andwithin the context of this disclosure, surfaces having contact angles inexcess of 170 degrees are referred to as superhydrophobic. As will befurther explained hereinbelow, the present invention, in one embodiment,provides for a nanotextured surface having a fibrillar coating of awater-repellant material. The fibrillar, nanotextured nature of thecoating of the present invention causes the surface to besuperhydrophobic.

As mentioned above, the nanotexture of surfaces can influence propertiesother than, or in addition to, their wettability by water. As will befurther explained hereinbelow, the present invention, in other aspects,may be utilized to prepare coated surfaces which are stronglyhydrophilic and/or oleophobic, or oleophilic. Also, the presentinvention may be utilized to prepare surfaces having controlled opticalproperties such as reflectivity and absorption. In further aspects ofthe present invention, chemical reactivity of the surfaces may becontrolled by utilizing the fibrillar, nanotextured materials of thepresent invention. All of these embodiments and advantages of theinvention will be apparent from the drawings, discussion and descriptionwhich follow.

BRIEF DESCRIPTION OF THE INVENTION

Disclosed is a method for providing a fibrillar, nanotextured coating ona substrate. In that regard, the substrate is coated with a reactionmixture which comprises a reagent having at least two reactive sitesthereupon. The reagent is hydrolyzable so as to provide a cross-linkedreaction product. The reaction mixture further includes a first solventwhich solvates the reagent and swells the reaction product. At least aportion of the reagent in the reaction mixture is hydrolyzed so as toprovide a cross-linked reaction product which is bonded to thesubstrate. A second solvent is mixed into the reaction mixture after thestep of hydrolyzing the reagent. The second solvent is miscible with thefirst solvent and is a non-solvent for the reaction product. This stepcauses the cross-linked reaction product to phase separate from thereaction mixture as a fibrillar, nanotextured coating which is bonded tothe substrate.

In specific embodiments, the reagent is a silane material such as amaterial of the formula:

R_(n)SiX_(4-n)

wherein n is in the range of 0-3; R is independently, one or more of Hor alkyl; and X is independently, one or more of a halogen or ahalogen-like species such as OSO₂—CF₃. Specific reagents of this typeinclude CH₃—Si—Cl₃, (CH₃)₂—SiCl₂, (CH₃)₃—SiCl and SiCl₄, used singly orin various combinations. In specific instances, the first solvent is anaromatic solvent such as benzene, toluene, or xylenes, and the secondsolvent is an alcohol.

In a further embodiment of the invention, the mixture of the first andsecond solvents is removed from the coated substrate, and this may beaccomplished through the use of a third solvent which is miscible withthe second solvent and is a non-solvent for the coating. In someinstances, the third solvent is water.

In a further aspect of the invention, the fibrillar, nanotexturedcoating prepared in accord with the foregoing is reacted with a firstconversion reagent which chemically alters at least a portion of thecoating. In some instances, this reagent may be an oxidizer such as anoxygen plasma, which converts the coating to a silica coating. Suchsilica coatings are typically very hydrophilic. The thus-reactedcoatings may be further reacted with a second and subsequent conversionreagent to further control their properties. For example, thesilica-based coating produced by oxidation can be reacted withappropriate fluoroalkyl materials to prepare a surface which is highlyoleophobic.

Also disclosed herein are coatings prepared by the methods of thepresent invention, including superhydrophobic coatings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a scanning electron micrograph of a coating of the presentinvention; and

FIG. 2 is an enlarged view of the coating of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

This invention is directed generally to nanotextured surfaces having afibrillar structure. Within the context of this disclosure, ananotextured surface is understood to be a surface having features inthe nanoscale range, typically a range of 5 nanometers to 1 micron. Afibrillar structure is understood to be a structure characterized by thepresence of a plurality of fibrous features, said fibers having acolumnar structure wherein the length of the column is greater than itsdiameter. The fibrillar structure may comprise a felted mat, separatedfibers having one end thereof anchored to the surface, or a mixture ofsuch textures. As discussed above, the nanotextured nature of thesurface enhances or otherwise modifies the physical and chemicalproperties of the material comprising the coating. These properties caninclude, among other things, chemical reactivity, wettability by oils orwater, and optical properties such as reflectivity and light absorption.

In the process of the present invention, a reagent is cross linkedproximate a substrate which is to be coated so as to form a cross-linkednetwork. This reaction is carried out in a solvent which, in addition tosolvating precursor materials, solvates and hence swells thecross-linked structure which is anchored to the substrate. In asubsequent step of the invention, the initial solvent material isreplaced with a second solvent which is a non-solvent for thecross-linked material. The second solvent is typically miscible with thefirst solvent, and this extraction process is carried out by adding thesecond solvent to the initial reaction mixture to induce phaseseparation. Given that the cross-linked network is anchored to thesubstrate, this phase separation produces the nanotextured fibrillarstructure which characterizes the present invention. This second solventmay subsequently be extracted by the use of a third solvent.

The thus produced nanotextured, fibrillar coating may be used in anas-is form, or it may be further reacted so as to modify its surfaceproperties. The thus modified coating may be further reacted so as toselectably control its surface properties.

The methodology of the present invention allows for the rapid andreliable production of nanoscale coatings on a wide variety of surfacesincluding metals, ceramics, glasses, polymers, textiles, paper stocks,mineral materials, as well as on natural surfaces such as wood, leather,and the like.

In one particular group of embodiments of the present invention, thenanotextured coatings are based upon silicon containing reactive speciessuch as silanes typically include a number of readily reactable sitesthereupon, which allow them to readily bond to a number of substratematerials and react so as to crosslink to other silane molecules. Oneparticular class of silane materials which may be used in the presentinvention are of the general formula:

R_(n)SiX_(4-n)

wherein n is in the range of 0-3; R is independently, one or more ofhydrogen or an organic group such as an alkyl (including substitutedalkyls); and X is independently, one or more of a halogen or ahalogen-like species such as OSO₂—CF₃.

Chlorosilanes are one specific group of materials which may be used inthe present invention, and organochlorosilanes such asmethylchlorosilanes are some specific members of this group. Thesematerials may be used either singly or in combination, and it will beapparent to one of skill in the art that the properties of thecross-linked network formed by the hydrolysis of these materials may becontrolled by controlling the ratio of different materials in a reactionmixture. SiCl₄ may be added to the reaction mixture to enhance crosslinking or otherwise control the properties of the hydrolyzed product.

In the process of the present invention, the hydrolyzable compounds suchas the silane are dissolved in a material which is a good solvent forthe reacting chemicals, which solvent also solvates and swells theresultant cross-linked network. Typical solvents include aromaticmaterials such as benzene, toluene, and various xylenes. The substrateis contacted with a reaction mixture comprising the hydrolyzable reagentand solvent, and the reagent is hydrolyzed, typically by including asmall amount of a hydrolyzing agent such as water in the reactionmixture. This causes the formation of the cross-linked network whichnetwork is anchored to the substrate. Reaction conditions will dependupon the specific nature of the reagents and the degree of crosslinking, and hence the ultimate structure of the nanotextured coating,which is desired. However, in some typical embodiments, the reactionmixture is approximately 0.1-2.0 molar with regard to the hydrolyzablereagent; although, the reaction mixture may be 5 or more molar withregard to the reagent.

Following the step of hydrolysis, the first solvent is extracted fromthe reaction mixture and replaced with a second solvent which is anon-solvent for the cross-linked reaction product. This extraction istypically carried out by mixing the second solvent into the reactionmixture following the step of hydrolysis. Typically, the solvent mixtureis then removed, and the coated substrate washed with at least one moreportion of the second solvent. In some instances, the second solvent isthen removed by washing with a third solvent. In one typical group ofembodiments, the second solvent is an alcohol such as ethanol orisopropanol. The third solvent, in such instances, if employed, maycomprise water. In some instances the majority of the reactive silanesolution is removed from the reaction flask before extraction with thesecond solvent, and in some instances the sample is rinsed with thefirst solvent before being extracted with the second solvent.

The thus described process produces a nanotextured fibrillar coating ofa silicon-based fibrous material on the surface of the substrate. Thiscoating is highly hydroscopic and, as will be explained hereinbelow,exhibits advancing and receding contact angles for water of more than170 degrees, and in some instances, more than 175 degrees. In particularinstances, both the advancing and receding contact angles for water are180 degrees making the surface perfectly hydrophobic. Coatings of thepresent invention are thus characterized as superhydrophobic.

FIG. 1 is a scanning electron micrograph of a coating prepared in accordwith the foregoing procedure. As will be seen, the coating is a highlyfibrillar structure, comprised of a plurality of filaments, each havinga length significantly exceeding its diameter. These filaments areanchored to the underlying substrate; and in some instances they may becross linked to one another, as is best seen in FIG. 2, which is anenlarged view of the coating of FIG. 1. The filaments form ananofeatured network. As such, the coatings of the present invention aredifferentiated from hydrophobic coatings of the prior art, and thisdifference is manifest by the fact that coatings of the presentinvention are superhydrophobic.

The properties of the coatings of the present invention may be furthermodified by chemical reaction. For example, the coating may be reactedwith additional silane materials. The coating may also be reacted withoxidizing agents such as an oxygen plasma; and this reaction willconvert at least a portion of the coating to silica which will cause thecoating to be hydrophilic. Such an oxidation reaction may be carried outeither prior to or subsequent to further couplings with silanes. In someinstances, the thus reacted surface may be further reacted with speciessuch as a fluorosilane to render them oleophobic. In yet otherinstances, the nanotextured surface may be reacted with dyes,fluorescent reagents, organometallic compounds, or other reagents so asto modify their surface properties. In view of the teaching presentedherein, yet other such surface modifications will be apparent to thoseof skill in the art.

The present invention will be described with reference to one particularprocess for preparing an ultrahydrophobic surface on a silicon wafer. Inthe process, silicon wafers were submerged in a 1.0 M solution ofCH₃SiCl₃ in toluene at room temperature for three hours. The hydrolysisreaction was carried out in vessels which were closed to the air duringthe reaction time, but exposed to relative humidity of approximately40-65% during solution and sample introduction, and this residual waterwas active to hydrolyze the slime compound. Thereafter, the wafers wererinsed with a further portion of toluene, rinsed with ethanol, rinsedwith an ethanol-water mixture and subsequently rinsed with water. Thesubstrates were then dried at 120° C.

Surfaces coated by the foregoing method are highly hydrophobic. Waterdroplets do not come to rest on the surfaces. Contact angle as measuredwith regard to a receding water droplet (θ_(R)) is 180 degrees. Thedroplet can be “pushed onto” the surface and the finite advancingcontact angle (O_(A)) is in the range of 175-178 degrees.

Given the highly hydrophobic nature of these surfaces, a new method formeasuring hydrophobicity was devised. In this method, surfaces to beexamined were lowered onto a supported water droplet and repetitivecontact, compression and release of the droplet were recorded by video.Surfaces having contact angles of less than 180 degrees exhibit someaffinity for the droplet during attachment and release; however, trulyhydrophobic surfaces will have a contact angle of 180 degrees andexhibit no affinity.

Coated surfaces prepared by the foregoing method are indistinguishableby eye from unmodified wafers, and in that regard contain no micronscale topography. Scanning electron micrography indicates that thecoating is comprised of a network of cylindrical fibers having diametersof approximately 40 nm. The method of the present invention promotesvertical polymerization of the silane onto a covalently attachedtoluene-swollen three-dimensional methylsiloxane network. Phaseseparation occurs during the ethanol rinse. In an experimental seriescomprising 100 repetitions of the foregoing procedure, extremelyhydrophobic surfaces are always formed. Perfectly hydrophobic surfaces(θ_(A)/θ_(R)=180 degrees/180 degrees) are formed in approximately 70% ofthe cases.

As discussed above, other silane compounds, including blends of silanecompounds, may also be used in a similar manner. While the foregoingexample employs chlorosilanes, good reactivity has also been foundutilizing iodosilanes as well as silanes based uponmethyltrifluorosulfonate.

Surfaces prepared according to the foregoing may be further modified.For example, exposure of the foregoing superhydrophobic surfaces to anoxidizing reagent such as an oxygen plasma converts at least some of themethylsilicone moieties to silica without a loss of nanoscopicmorphology. The coatings thus modified are spontaneously wetted bywater. The silica surfaces thus produced may be still further modified.For example, treatment of the silica surfaces with fluoroalkyl silanesproduces oleophobic surfaces that are not wet by hydrocarbon liquids.

In one group of surface modification reactions, samples of thenanotextured coating were treated with an oxygen plasma, as describedabove, and the samples were introduced into a reaction flask and treatedwith a toluene solution (0.1-2.0 molar) of a variety of silanes for onehour. Alternatively, the samples could be exposed to reactive silanes inthe vapor phase. The silanes used were of the type RSiX₃, R₂SiX₂ andR₃SiX where R is one or more of alkyl, aryl, fluoro alkyl or aminoalkyl, and X is one or more of Cl, N(R)₂ or OSO₂CF₃. The samples wereisolated and rinsed (in this order) with 2×10 ml of toluene, 3×10 ml ofethanol, 2×10 ml of ethanol-water (1:1), 2×10 ml of water, and thendried in a clean oven at 120° C. for 10 minutes. Silanes whereinR═CH₂—CH₂—C₆F₁₃ and CH₂—CH₂—C₈F₁₇ were found to render surfaces thatwere perfectly hydrophobic (advancing and receding contact angles of 180degrees) and also repellant to hydrocarbon liquids (oleophobic).

The present invention provides methods and materials for disposing afibrillar, nanotextured coating onto a variety of substrate surfaces.The properties of the coating may be tailored to affect its wettabilityby water, hydrocarbons and other materials. Likewise, the opticalproperties of the surface may be readily controlled, as for example withregard to reflectivity, light absorption, fluorescence and the like. Inview of the teaching presented herein, numerous modifications andvariations of the invention will be readily apparent to those of skillin the art. The foregoing drawings, discussion and description areillustrative of specific embodiments of the invention, but are not meantto be limitations upon the practice thereof. It is the following claims,including all equivalents, which define the scope of the invention.

1. A substrate having a fibrillar coating thereupon: said coatingcomprising the hydrolysis product of a silane, said coating having afibrillar structure comprised of a plurality of nanofibers having oneend thereof bound to said substrate.
 2. The coating of claim 1, furthercharacterized in that its contact angle with regard to water is at least170 degrees.
 3. The coating of claim 2, wherein said contact angle is atleast 175 degrees.
 4. The coating of claim 1, wherein at least 70% ofsaid nanofibers have a diameter in the range of 20-70 nanometers.
 5. Thecoating of claim 1, wherein said nanofibers are configured as a networkof cross-linked fibers.
 6. The coating of claim 1, wherein said silaneis of the formula:R_(n)SiX_(4-n) wherein n is in the range of 0-3; R is independently H oralkyl; and X is independently halogen or OSO₂—CF₃.
 7. A superhydrophobiccoating, said coating being characterized in that the advancing andreceding angles of contact thereof, with regard to water, are at least179 degrees.